Again, "Every Rock Formation" doesn't specifically mean that it has it's own spirit. Only that it is protected by a spirit.It's quoted from the 2e Darksun book. Every rock formation. I think it's safe to assume that there are greater and lesser spirits, and the druids would serve the greater ones.
Though there is another thing to note: "Unique geographic features are guarded by spirits when druids serve" Not "That" druids serve. Or "Which" druids serve. -When- Druids serve.
Does the Druid's presence somehow impart genius loci? Does their oath to protect a unique geographic feature result in a spirit traveling to that location to guard it?